High-voltage batteries for hybrid or electric-powered vehicles are often composed of series connected single cells. These are manufactured as nickel-metal hydride (Ni-MH), lead or lithium ion (Li-ion) based cells, for example. In the case of Li-ion cells, the nominal voltage is approximately 3.6 V; the final charging voltage is 4.2 V. Voltages of over 600 V are generated by implementing a series connection. Connecting single cells in series can lead to a failed or weakened cell affecting the entire battery stack. To monitor such high-voltage and thus safety-critical batteries, battery control units are used in hybrid and electric-powered vehicles whose purpose is to protect the individual cells from overcharging, overdischarging and thermal overloading, and to prolong the lifetime thereof. To this end, efforts are continually directed towards adapting all cells to the same charge condition. In addition, the battery control unit must estimate the remaining battery capacity from the available parameters and make the calculated value available to the higher-level hybrid control unit. The communication is normally carried out via the vehicle CAN (controller-area network) bus.
The structure of a battery control unit is organized into the actual battery management system (BMS) and the cell modules (CM). They are interconnected via an isolated CAN bus. In each case, a CM is assigned to a cell stack, which is a subset of all single cells of the battery and is responsible both for measuring the cell voltages, as well as for selectively discharging individual cells. For this purpose, the CM has a switch (transistor) for each cell that, in the ON state, connects the cell to a load via a resistor.
By activating the associated switch, the control unit always loads those cells which have a higher voltage level than the remaining cells. This mechanism of cell charge compensation has the effect of keeping all cells of the battery at the same charge level. Differences in cell behavior are thus evened out.
Generally, to ensure proper control unit functioning, different types of tests are performed at different development stages. If hardware and software are already available for a control unit, what are known as hardware-in-the-loop (HIL) tests are normally carried out, in which the presence of a controlled system, as well as possibly of other external components or other control units, is simulated for the control unit to be tested. In the case of a motor controller, the controlled system is the motor, for example, or in the present instance of a battery control unit, it is the battery. External components can be other control units, for example, to which the control unit to be tested is connected via a bus and with which it communicates via messages. Such control units are often simulated by what is generally referred to as a restbus simulation, in which only those messages to be expected from the control unit to be tested are simulated on a bus.
Generally, the HIL simulation can take place at different levels:                At the signal level, only the digital signals are computed in a simulation model on a suitable processing unit, for example a real-time processor, and transmitted to the processing unit of the control unit. This requires access to a suitable signal interface. Often, this necessitates opening the control unit to be tested, particularly when testing control units for electric motors. Such tests are relatively simple to carry out, as soon as the signal interface is exposed. However, they cannot be used to verify the reliability performance of the power electronics. Real currents and/or voltages, which are generated in accordance with a simulation model, are transmitted at what is generally known as the “power level” directly via the power electronics of the control unit to the same. This is also referred to as emulation when generating these voltages and currents.        At this test level, not only is the control algorithm of the control unit tested, but also the power electronics thereof. Since the controlled system is only simulated, it is readily interchangeable, and the tests can be flexibly adapted to different situations.        When simulation is performed at the mechanical level, the complete unit, composed of the control unit and the physically present controlled system, can be tested on a mechanical test bench using an electric motor or a throttle valve, for example. These tests are very expensive, rather inflexible and, to some extent, also safety critical. However, they permit testing of load conditions in real-world operation.        
Fault simulations used for testing the reaction of the control unit to fault situations make up another important component of HIL tests. To simulate cable ruptures or similar faults, additional plug-in cards for HIL systems, generally referred to as failure insertion units (FIUs), are normally available. They encompass circuits having switches that are controllable remotely and by automation to simulate cable ruptures, short circuits and/or mistakenly interchanged terminals for all control unit connections. There are FIU cards both for sensors, as well as for actuators; they being additionally combined with load cards, in the case of actuators.
Only tests for battery control units are discussed in the following. In this context, merely the control strategy of the BMS is to be examined; thus, it suffices to test only the BMS. In this case, the CM are simulated at the signal level. However, to test the entire battery control unit at the power level, all or at least one CM must be integrated in the HIL system. As a controlled system, it is necessary to include both a real-time battery simulation model, as well as a cell voltage emulator for outputting the analog terminal voltage. Battery simulation models and cell voltage emulators are commercially available. Examples of commercially available battery simulation models may be found, for example, in “Electric Drive Technology at dSPACE,” a brochure published by dSpace and available on the Internet at dspace.de/shared/data/bkm/ElectricalDrive_en/blaetterkatalog/.
These battery simulation models for testing battery control units simulate the battery behavior as an interconnection of a plurality of single cells. In this context, the cell model reproduces the cell voltage and the charge state of a battery cell. The typical cell behavior of different cell technologies, such as Li-ions, Ni-MH or lead, can be taken into consideration. This includes differences in charging and discharging, as well as the dynamic performance in response to loading step changes, and residual currents caused by gassing effects, for example. Normally, the battery simulation model also provides one or a plurality of temperature values that are simulated for the control unit, for example also via the hardware unit of the cell voltage emulators. The battery model is then composed of individual cell models. In this context, it supports the series connection of cells to reach the required voltage level, as well as a parallel connection and the currents resulting therefrom. Individual cell parameters and states, such as internal resistance or initial charge state, remain individually settable in the process, and the resulting cell voltages can also be made available to the battery control unit on an individual basis. The currents adjusted by the battery control unit for the cell charge compensation are then likewise considered. In the process, parameterization may be handled by graphical user interfaces. An example of such a graphical user interface is the ModelDesk program referenced at page 315 of “Catalog 2010” published by dSPACE, available at dspace.de/shared/data/bkm/catalog2010/blaetterkatalog/.
Cell voltage emulators are normally composed of individual, controllable voltage sources, whose setpoint voltage values are determined by the battery model. A cell voltage emulator is often modularly constructed by interconnecting a plurality of emulation units which each include a group of cell emulation channels having a separate, controllable voltage source. To supply power to an emulation unit, a power supply unit is used, whose voltage is distributed by DC/DC converters to emulate an individual battery cell per cell emulation channel. The channels are galvanically isolated from one another and are connectable in series, as well as in parallel. Presently, total voltages of nearly 1000 V are reached in the case of a series connection. Higher currents can be produced in the case of the parallel connection. Besides the voltage supply of a few volts, a single emulation channel includes an amplifier unit for controlling the cell voltage in accordance with the predefined setpoint value. A relatively broad voltage range for a single emulation channel permits emulation of defective cells. For example, a short-circuited cell can be emulated by outputting 0 V. On the other hand, a voltage greater than the nominal voltage emulates an increased internal resistance of the cell during the charging process. In order to realistically emulate a battery, it is necessary to rapidly correct setpoint value step changes. Currently requirements stipulate that this take place in less than 500 μs.
The requisite, fast closed-loop control of the voltage sources is mostly performed by a control unit, i.e., a fast processing unit. For this purpose, an FPGA (field-programmable gate array) may be provided, for example, which is able to control the channels of a plurality of emulation units. The setpoint voltage values are transmitted (digitally) to the amplifier units of the individual emulation channels via a bus connection having a galvanically isolated interface to the channels, for example through optocouplers. The control unit exchanges data with a higher-order processing unit on which the battery simulation model is executed in accordance with which the setpoint voltage values are determined for the battery emulation. As in the case of a battery in real-world operation, the emulated cell voltages are also connected in series in the emulation process.
The voltage is measured in the control unit, i.e., in the CM, with a high degree of precision since the battery cells often have a very flat discharge curve. For this reason, a high degree of precision is required when emulating the cell voltages. Deviations higher than 2 mV are frequently not tolerable. The cell charging compensation function loads the emulated voltage source with several hundred mA. The accuracy of the voltage must be retained during loading; it is, therefore, necessary to compensate for a voltage drop on the lines from the emulation to the control unit. For this purpose, each cell emulation channel is provided with a measurement line which picks off the exact voltage value at the input of the CM for the amplifier unit. Moreover, in each cell emulation channel, the compensation current is recorded, transferred to the control unit, and taken into consideration in order to correctly simulate the charge state. Another measurement line is used to examine whether the voltage value of the reference potential, which, in the case of the series connection of the cells and channels is defined by the potential of the voltage source connected in incoming circuit, also conforms with the voltage value that the control unit records for the voltage source connected in incoming circuit.
However, a disadvantage of the afore-mentioned battery simulation systems is that they are not capable of simulating faults, such as cable ruptures, etc., since the high voltages and the additional measurement line required in battery emulation systems preclude the use of the failure simulation cards (FIU cards) normally used in HIL simulations.